A prosthetic device helps restore mobility to people who lack able-bodied motion or gait. The prosthetic device is intended to replace the function or appearance of a missing limb and can return mobility to the wearer or user. The prosthetic device is available to replace or support various portions of the body. A lower limb prosthetic device includes, for example, the prosthetic foot, foot-ankle prosthesis, prosthetic knee joint, and prosthetic hip joint. People who require a lower limb prosthesis often expend more metabolic power to walk or move at the same speed as an able-bodied individual. One goal of the lower limb prosthetic device is to help the user achieve a normal gait while reducing energy expended by the user.
Prosthetic devices can be divided into two groups, passive devices and active devices. A passive lower limb prosthetic generally relies on a compliant member, such as a spring, to store and release energy. A spring is able to return no more than the amount of energy that is put into the spring. Thus, the energy that is released by a spring in a passive device is limited to the energy as is put in by the user. For example, a spring-based passive foot prosthetic provides about half of the peak power required for gait. The user of a passive device must expend additional energy through other muscles and joints to maintain a normal walking gait. Therefore, the passive prosthetic design is limited in capacity to help users reduce metabolic energy expenditure while achieving a normal walking gait and performing other activities.
An active device differs from the passive device in that the active device uses a motor to supply power to the device and to control the device. Some active device designs are inefficient, either requiring relatively large motors, which are heavy and undesirable for wearable devices, or providing low peak power output, which is insufficient for many activities. Control systems for the active device are limited in capability to control active devices. The active prosthetic is typically restricted to a single degree of freedom, which reduces the motion available to the device. Further, the active prosthetic may be limited to low power activities, because the power necessary for high power activities is unattainable in a small portable system. One goal of the active prosthetic device is to increase efficiency of the active components and to build a lighter weight device.
Prosthetic devices are typically designed for a specific activity, such as walking. The majority of active compliant devices utilize a traditional rigid structure. The traditional rigid structure typically includes links powered by actuators such as electric motors or hydraulics. One strategy employs an architecture having a joint which is powered by a compliant member, such as a spring, and an active member, such as a motor driven screw, arranged in series. An activity-specific design strategy and traditional rigid structures may be suited for one specific activity, but the designs are limited in application and are not efficient beyond the intended activity. For example, devices designed for walking perform poorly for running, navigating uneven terrain, walking up and down inclines or stairs, or simply balancing while standing. Carrying heavy loads or transitioning from walking to running remains a challenge for users. Some active devices are ineffective for activities requiring both high velocities under low load and low velocities under high load.